This invention relates generally to microwave monolithic integrated circuits and more particularly to monolithic lumped element circuit networks.
As it is known in the art, so-called microwave monolithic integrated circuits (MMICs) include active and passive devices which are formed over suitable semiconductor substrates using semiconductor integrated circuit techniques to provide various types of microwave circuits. Amongst the circuits provided are filters, amplifiers, phase shifters, switches, and the like. Often such circuits are integrated together to form higher order integrated functions. A common network employed in such integrated circuits are lumped element networks such as a lumped element RLC or LC network used to provide impedance matching, filtering, or bias networks for an amplifier, phase shift elements for a phase shifter, or other circuit elements.
Generally with lumped element networks, it is conventional to form the inductor elements of the networks as distributed or lumped element equivalents of an inductor. That is, it is common to use a strip conductor formed on a dielectric substrate having a controlled impedance to provide an inductor having a selected inductance at microwave frequencies. Similarly, it is also conventional to form metal insulator metal (MIM) capacitors on such a substrate and integrate such capacitors with the sections of strip conductor to form various combinations of inductors and capacitors.
It is also known that many applications of such MMICs have a requirement for low power operation or consumption. This requires the use of higher circuit impedances for transmission lines and other elements. In a filter, for example, in which low power consumption is a requirement, the value of inductance for the inductors scale directly with the characteristic impedance desired for the filter, whereas the values of capacitance for the capacitors are inversely proportional to the desired characteristic impedance. With such a circuit requirement therefore, it is often the case that the desired values of the capacitance required for a filter are very small. For example, it often arises that the required capacitance is not significantly different from the parasitic capacitance which is inherent in the particular circuit. This situation presents problems both for modelling the circuit, as well as, fabricating the circuit, since the small amount of capacitance needed makes practical fabrication difficult since tight tolerances must be held in the capacitors.
Generally, the approach used to solve this problem has been to compensate for the parasitic capacitance by using smaller shunt MIM capacitors in the filter so that the sum of the MIM capacitors and parasitic capacitance is the desired value. This compensation approach, however, is fraught with many problems. For example, the exact value of the parasitic capacitance is difficult to determine and may only be finally quantified after a design has been processed into a fabricated circuit. Moreover, using the parasitic capacitance of the filter in this manner may provide a requirement for a discrete capacitor having a very small value of capacitance which might be even more difficult to provide particularly within fixed process design rules as used throughout the industry.